Imaging molecular structure of channels and receptors with an atomic force microscope.

Abstract

Biological membranes contain specialized protein macromolecules such as channels, pumps and receptors. Physiologically, membranes and their constituent macromolecules are the interface surfaces toward which most of the regulatory biochemical and other signals are directed. Yet very little is known about these surfaces. The structure of biological membranes has been analyzed primarily using imaging techniques that are limited in their resolution of surface topology. An atomic force microscope (AFM) developed by Binnig, Quate and Gerber, can image molecular structures on specimen surfaces with subnanometer resolution, under diverse environmental conditions. Also, AFM can manipulate surfaces with molecular precision: it can nanodissect, translocate, and reorganize molecules on surface. The surface topology has been imaged for several hydrated channels, pumps and receptors which were a) present in isolated native membranes, b) reconstituted in artificial membrane or, c) expressed in an appropriate expression system. These images, at molecular resolution, reveal exciting new findings about their architecture. AFM induced "force dissection" reveals surfaces which are commonly inaccessible. In whole cell studies, in addition to the molecular structure of membrane receptors and channels, correlative electrical and biochemical activities have been examined. Such study suggests a "single cell" experiment where the structure-function correlation of many cloned channels and receptors can be understood.